Add daily OpenAPI PR reviewer agentic workflow

Adds a gh-aw (GitHub Agentic Workflow) that runs the openapi-pr-reviewer logic daily: reviews the latest "Update OpenAPI 3.0/3.1 Descriptions" PRs, merges them and closes older superseded PRs when there are no breaking changes, and notifies #api-platform on Slack. Ships in staged (dry-run) mode until secrets and branch-protection bypass are provisioned. Tracked in github/api-platform#3408.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jencarlucci